Semiconductor device

ABSTRACT

A synchronous DRAM is provided which includes arrangements for operations of power supply circuitry based upon whether the DRAM is in a power down mode or not. In one embodiment, a first power supply circuit and a second power supply circuit are provided which both receive externally supplied voltages and output internal supply voltages. The first power supply circuit is not in operation when a semiconductor device of the synchronous DRAM is in a power down mode. However, the second power supply circuit is continuously in operation during the power down mode. In another arrangement, the operation of a voltage limiter circuit is controlled based on whether or not the DRAM is in a power down mode.

CROSS-REFERENCE TO RELATED APPLICATION

This application is the first, of two concurrently filed applicationswhich are continuations of U.S. application Ser. No. 11/483,649, filedJul. 11, 2006, which, in turn is a continuation of U.S. application Ser.No. 11/183,802, filed Jul. 19, 2005 (now U.S. Pat. No. 7,082,074),which, in turn is a continuation of U.S. application Ser. No. 11/084,138(now U.S. Pat. No. 7,032,202); which, in turn, is a continuation of U.S.application Ser. No. 10/724,781 (now U.S. Pat. No. 6,870,790), filedDec. 2, 2003, which, in turn, is a divisional of Ser. No. 10/259,579(now U.S. Pat. No. 6,680,875), filed Sep. 30, 2002, which is adivisional of application Ser. No. 09/759,244, filed Jan. 16, 2001 (nowAbandoned); which is a continuation of Ser. No. 09/289,660, filed Apr.12, 1999 (now U.S. Pat. No. 6,195,306), the entire disclosures of whichare hereby incorporated by reference.

BACKGROUND OF THE INVENTION

The present invention relates to power supply circuits of asemiconductor memory and, more specifically, to a control method forachieving low power dissipation.

In this specification, reference will be made to the followingpublications: Japanese Patent Laid-Open No. 105682/1995 (called theCited Reference 1; corresponding to U.S. Pat. No. 5,463,588), andJapanese Patent Laid-Open. No. 161481/1997 (called the Cited Reference2).

Semiconductor memories extensively utilize what is known as an on-chipvoltage limiter method (i.e., power-down method) whereby thesemiconductor chip lowers an externally supplied voltage to generate aninternal voltage for use as a power supply. The method is used to reducepower dissipation of circuits or to improve reliability of fine elementsin the device. In achieving such objects, voltage limiter circuits(power-down circuits) are utilized to generate the internal supplyvoltage.

A voltage limiter circuit consumes a steady current so as to maintain anoutput voltage level even when the semiconductor memory is in standbymode. As a way to reduce power dissipation in the standby state, theCited Reference 1 proposes a total of eight voltage limiter circuits,i.e., two limiter circuits furnished to each of four memory cell,arrays; and a single, common voltage limiter circuit that is common toall memory cell arrays (FIG. 3 in the Cited Reference 1). The commonvoltage limiter circuit is constantly in operation. The eight voltagelimiter circuits start operating simultaneously when the memory isaccessed, and four of the circuits are allowed to remain active uponelapse of a predetermined period of time following the start of theaccess.

The Cited Reference 2 discloses first and second voltage limitersfurnished corresponding to respective first and a second banks, alongwith a description of operation timings of the limiters. When the firstbank is ordered to be activated, the first voltage limiter generates aninternal voltage. If the second bank is ordered to be activated whilethe first bank is still active, the second voltage limiter alsogenerates an internal voltage in cooperation with the first Voltagelimiter (FIG. 12 of the Cited Reference 2).

SUMMARY OF THE INVENTION

The inventors of the present invention have studied the powerdissipation of SDRAMs (synchronous dynamic random access memories) inactive standby mode. The active standby mode of the SDRAM is a mode inwhich a memory bank is left active to retain one-word data in senseamplifiers in preparation for memory access, with a read or a writecommand yet to be issued. Whereas it takes a relatively long time toread data from dynamic memory cells, the data, once placed in senseamplifiers, may be read at high speed because the data thus retained arehandled as if they were in a column of an SRAM (static random accessmemory).

Active standby mode is entered when a bank active command is applied.The application of the bank active command selects a word line andoperates sense amplifiers, thus causing a large current to flow. Thenwith no further command issued, no power dissipation should occur intheory. In practice, where a voltage limiter circuit is included, acurrent flows to that circuit. If both standby and operating voltagelimiter circuits operate, they dissipate a fairly large current(generally of several to tens of mA). In particular, many synchronousDRAMs are arranged to leave their banks active to take advantage of thehigh-speed data transmission feature. This means that an active standbycurrent can have a significant effect on the power dissipation of thesystem as a whole.

In the disclosure of the Cited Reference 1, bank-related operationsspecific to SDRAMs are not considered. No technique is disclosed inconnection with controlling the driving capability of voltage limitersin units of a plurality of memory arrays, memory blocks or banks. Theinventors of this invention found that if the techniques of the CitedReference 1 were applied to SDRAMs, as many as eight voltage limitercircuits (nine if a common voltage limiter circuit is included) wouldoperate simultaneously in the initial stage of an active period, causingan excessively large operation current to flow at peak time. With theinitial stage ended and with the active period still in effect, fourvoltage limiter circuits (five if the common voltage limiter circuit isincluded) would be in operation. This, the inventors found, will giverise to an unnecessarily high level of power dissipation during theactive period.

The Cited Reference 2 does not take the active standby mode of SDRAMsinto consideration. The inventors of this invention found thatsuccessively activating a plurality of memory banks in the SDRAM wouldcause a growing number of the corresponding voltage limiter circuits tobecome active; and putting a plurality of memory banks in active standbymode would unnecessarily add up operation currents of the correspondinglimiter circuits. The increase of such operation currents can become aserious problem if a large number of banks are involved.

It is therefore an object of the present invention to overcome theabove-described drawbacks and disadvantages and to provide asemiconductor device comprising: first and second memory banks activatedby first and second commands (first and second control signals)respectively; a power supply line for supplying a predetermined voltageto the first and the second memory banks; and first and second powersupply circuits (first and second voltage generating circuits) havingrespective output nodes connected to the power supply line, the firstand the second power supply circuits generating the predeterminedvoltage; wherein the first power supply circuit starts generating thepredetermined voltage in response to the first command; and wherein,upon input of the second command with the first memory bank left active,the first power supply circuit stops generating the predeterminedvoltage in response to the second command while the second power supplycircuit starts generating the predetermined voltage in response to thesecond command.

Other objects, features and advantages of the invention will become moreapparent upon a reading of the following description taken withreference to the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a synchronous DRAM practiced as a firstembodiment of this invention;

FIG. 2 is a diagram of a bank as part of the synchronous DRAM in FIG. 1;

FIG. 3 is a detailed circuit diagram of part of the bank in FIG. 2;

FIG. 4 is a block diagram of power supply circuits in the synchronousDRAM of FIG. 1;

FIG. 5 is a circuit diagram of a voltage limiter control circuit CTL0 inthe synchronous DRAM of FIG. 1;

FIG. 6 is an operation waveform chart applicable to the control circuitin FIG. 5;

FIG. 7 is another operation waveform chart applicable to the controlcircuit in FIG. 5;

FIG. 8 is a circuit diagram of a voltage limiter control circuit CTL1 inthe synchronous DRAM of FIG. 1;

FIG. 9 is an operation waveform chart applicable to the control circuitin FIG. 8;

FIG. 10 is another operation waveform chart applicable to the controlcircuit in FIG. 8;

FIG. 11 is another operation waveform chart applicable to the controlcircuit in FIG. 8;

FIG. 12 is another operation waveform chart applicable to the controlcircuit in FIG. 8;

FIG. 13 is a circuit diagram of voltage limiter circuits VDL0 throughVDL3 in the synchronous DRAM of FIG. 1;

FIG. 14 is a circuit diagram of voltage limiter circuits VCL4 throughVCL6 in the synchronous DRAM of FIG. 1;

FIG. 15 is a block diagram of a synchronous DRAM practiced as a secondembodiment of this invention;

FIG. 16 is a circuit diagram of a voltage limiter control circuit CTLOin the synchronous DRAM of FIG. 15;

FIG. 17 is an operation waveform chart applicable to the control circuitin FIG. 16;

FIG. 18 is a schematic view of a synchronous DRAM practiced as a thirdembodiment of this invention;

FIG. 19 is a block diagram of power supply circuits in the synchronousDRAM of FIG. 18;

FIG. 20 is a circuit diagram of a voltage limiter control circuit CTLOin the synchronous DRAM of FIG. 18;

FIG. 21 is an operation waveform chart applicable to the control circuitin FIG. 20;

FIG. 22 is a schematic view of a synchronous DRAM practiced as a fourthembodiment of this invention;

FIG. 23 is a block diagram of power supply circuits in the synchronousDRAM of FIG. 22;

FIG. 24 is a circuit diagram of a voltage limiter control circuit CTL1in the synchronous DRAM of FIG. 22;

FIG. 25 is an operation waveform chart applicable to the control circuitin FIG. 24;

FIG. 26 is a schematic view of a synchronous DRAM practiced as a fifthembodiment of this invention; and

FIG. 27 is a block diagram of power supply circuits in the synchronousDRAM of FIG. 26.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of this invention will now be described withreference to the accompanying drawings. Circuit elements constitutingeach of the blocks making up the embodiments are formed, but notlimited, by known integrated circuit techniques such as those of CMOS(complementary MOS) transistors on a single semiconductor substrate madeillustratively of single crystal silicon. Of the circuit symbolsrepresenting MOSFETs or MISFETs (metal insulation semiconductor FETs),those not arrowed denote N-type MOSFETs (NMOSs) and those arrowed standfor P-type MOSFETs (PMOSs).

First Embodiment

FIG. 1 shows a four-bank synchronous DRAM embodying the invention. InFIG. 1, CHIP stands for a semiconductor memory chip; BANK0 through BANK3denote the memory banks; CKT represents peripheral circuits common toall banks; VDL0 through VDL3 and VDLS indicate voltage limiter circuits(power-down circuits; more commonly, power supply circuits) forgenerating an internal supply voltage VDL (first internal voltage) foruse by memory arrays from an externally supplied voltage VDD; and VCL4through VCL6 and VCLS denote voltage limiter circuits (voltagegenerating circuits) for generating an internal supply voltage VCL(second internal voltage) for use by peripheral circuits from thevoltage VDD. Illustratively, the voltages VDD, VCL and VDL are 3.3 V12.2 V and 1.8 V respectively. The internal circuits are supplied withthe two internal voltages VCL and VDL which are derived from theexternally supplied voltage VDD and which are lower than the voltageVDD. The voltage VDL is kept lower than the voltage VCL.

The control circuit CTL0 controls the circuits VDL0 through VDL3, andthe control circuit CTL1 controls the circuits VCL4 through VCL6. Thecircuits CTL0 and CTL1 are included in the peripheral circuits CKT. Thecircuits CKT further comprise bonding pads, input buffers, outputbuffers, a main control circuit (to be described later), a mainamplifier and a refresh counter. The input and output buffers operateon, but are not limited by, the externally supplied voltage VDD.

FIG. 2 shows a typical bank structure. In FIG. 2, MA denotes a memoryarray in which memory cells are arranged in a matrix fashion; SC standsfor a sense circuit; SWD represents a sub-word line driver; and ISindicates an intersection region where the sense circuit and thesub-word line driver intersect. The region IS includes a sense amplifierdriving circuit, as will be described later.

The first internal voltage VDL is used to power the sense amplifierdriving circuit. To reduce parasitic resistance, the VDL line iscomposed of a second and a third metal layers arranged in a mesh manner.As depicted in FIG. 1, the VDL line is furnished across the memory banksBANK0 through BANK3. That is, the VDL line is provided commonly to BANK0through BANK3. The voltage VDL is also supplied to a VDL/2 generatingcircuit, not shown, which generates half the voltage VDL. As will bedescribed later, the voltage VDL/2 is used to precharge plate electrodesand bit lines of the memory cells.

The second internal voltage VCL is supplied to row decoders XD0 and XD1,column decoders YD0 and YD1, bank-dedicated peripheral circuits BCKT,and common peripheral circuits CKT. The bank-dedicated peripheralcircuits BCKT include an address latch and a pre-decoder. Because therow decoders, column decoders and peripheral circuits BCKT and CKToperate on the voltage VCL, the VCL line passes through these circuits.As shown in FIG. 1, the VCL line is also furnished across the memorybanks BANK0 through BANK3. Whereas the input and output buffers includedin the common peripheral circuits CKT operate on the voltage VDD, aswill be described earlier, the majority of the other circuits in thecommon peripheral circuits CKT operate on the voltage VCL that is lowerthan the voltage VDD so as to achieve low power dissipation.Illustratively, the main control circuit, main amplifier, refreshcounter, and control circuits CTL0 and .CTL1 operate on the voltage VCLas well.

FIG. 3 depicts details of a memory array MA, a sense circuit SC and asense amplifier driving circuit IS. At an intersection between asub-word line SWL and a bit line pair BL and /BL is a dynamic memorycell MC furnished in a known folded data line pair fashion, the memorycell comprising a capacitor connected to a source-drain path of a switchMOSFET. The other end of the capacity is a plate electrode suppliedcommonly with the voltage VDL/2. The sense circuit SC comprises a senseamplifier SA (made of two cross-connected CMOS inverters) for amplifyinga signal voltage on the bit line pair, and a bit line prechargingcircuit PC for precharging the bit line pair to half the voltage VDL.Paired signal lines CSN and CSP for driving the sense amplifier SA arewired up to the sense amplifier driving circuit. A precharging circuitCSPC similar to the bit line precharging circuit is furnished betweenthe signal lines CSN and CSP and is supplied with the voltage VDL/2. Thesense amplifier driving circuit IS comprises three MOS transistors MN1through MN3.

When not in amplifying operation, the signal lines CSN and CSP areprecharged by the precharging circuit CSPC to half the voltage VDL. Anamplifying operation is started by initially turning on the transistorsMN1 and MN2. This connects the signal line CSN to ground and the signalline CSP to the external power supply VDD, causing the line CSN to dropand the line CSP to rise in potential. When the potential of the lineCSP is raised close to the VDL level, the MOS transistor MN2 is turnedoff and the MOS transistor MN3 is turned on. The activated anddeactivated transistors connect the line CSP to the internal powersupply VDL. Eventually, the lines CSN and CSP reach 0 V and the voltageVDL respectively. This completes the amplifying operation, with one ofthe paired bit lines on the 0 V level and the other on the VDL level.The purpose of connecting the line CSP initially to the voltage VDDinstead of directly to the voltage VDL is twofold: to make theamplifying operation faster, and to ease burdens on the VDL voltagelimiter circuit. The technique is known as overdrive.

FIG. 4 shows connecting relations between power supply circuits of thefirst embodiment. The voltage limiter circuits VDL0 through VDL3 andVDLS generate the first internal voltage VDL by referring to a referencevoltage VRD generated by a reference voltage generating circuit RVG (notshown in FIG. 1). The circuit VDLS is continuously in operation. Becauseits current supplying capacity is small, the circuit VDLS consumes a lowlevel of power. The circuits VDL0 through VDL3 consume relatively highlevels of power, have enhanced current supplying capacities, and areturned on and off by active signals LD0 through LD3 generated by thecontrol circuit CTL0. In FIG. 4, the sizes of the blocks representingthe limiter circuits denote proportionally their current supplyingcapacities. The outputs of the circuits VDL0 through VDL3 and VDLS areconnected to the second layer metal line (indicated by solid lines inFIGS. 1 and 2) and the third layer metal line (denoted by dotted linesin FIGS. 1 and 2) arranged perpendicular to one another.

The voltage limiter circuits VCL4 through VCL6 and VCLS generate thesecond internal voltage VCL by referring to a reference voltage VRCgenerated by the reference voltage generating circuit RVG. The circuitVCLS is continuously in operation. Because its current supplyingcapacity is small, the circuit VCLS consumes a low level of power. Thecircuits VCL4 through VCL6 consume relatively high levels of power andhave enhanced current supplying capacities. The voltage limiter circuitVCL4 is turned on and off by an active signal LC4 generated by thecontrol circuit CTL1, and the circuits VCL5 and VCL6 are activated anddeactivated by an active signal LC5 generated by the circuit CTL1. Theoutputs of the circuits VCL4 through VCL6 and VCLS are interconnected.

The main control circuit CTLM (not shown in FIG. 1) receives a clocksignal CLK, a chip select signal /CS (a slash “/” prefixed to a signalname signifies that the signal is a complementary signal that is enabledwhen brought Low), a row address strobe signal /RAS, a column addressstrobe signal /CAS, a write enable signal /WE, and a clock enable signalCKE; it interprets commands; and it establishes operation modesaccordingly. Synchronous DRAMs have a standardized method fordesignating commands through the combination of such signals as /CS,/RAS, /CAS, /WE and CKE in effect at a leading edge of the clock signalCLK. The first embodiment of this invention obeys the standardizedmethod.

The first embodiment is characterized roughly by two features. That is,the outputs of a plurality of voltage limiter circuits areinterconnected, and the circuits are turned on and off individuallydepending on the operation mode the memory is in as described below.

The control circuit CTL0 will now be described with reference to thecircuit diagram of FIG. 5 as well as the operation waveform charts ofFIG. 6 and FIG. 7. Input signals BA0 through BA3 are bank active signals(control signals). Each of the signals is set to “1” when activating thecorresponding memory bank (one of banks 0 through 3). and to “0” whendeactivating it. A bank active command ACTV standardized for SDRAMs isdesignated by use of /CS=L, /RAS=L, /CAS=H, /WE=H, a row address (todesignate a word line), and a bank address (to designate a bank number).More commonly, a bank is activated (i.e., selected) by first designatingthe bank location and one word in the bank. The one-word data are thenread out to a plurality of corresponding sense amplifiers and latched.RF represents a refresh signal that is set to “1” when refreshing thememory. As described above, the output signals LD0 through LD3 turn onand off the VDL limiter circuits VDL0 through VDL3 respectively (set to“1” for circuit activation and “0” for deactivation).

FIG. 6 is an operation waveform chart applicable to the control circuitof FIG. 5 in normal operation. At times t0, t1 t2 and t3, commands foractivating banks 0 through 3 (ACTV0 through ACTV3) are inputrespectively. At time t4, an all-bank precharge command (PRE ALL) isinput. When the command PRE ALL is input, all banks are deactivated, andthe paired data lines and paired sense amplifier driving lines of eachbank are precharged to the voltage VDL/2. Although it is common practiceto input a read or a write command between the activation of a bank anda precharging operation, the command input is omitted here for purposeof simplification. When the input signal BA0 reaches “1,” an inverter100-0 and a one-shot pulse generator 101-0 act to set a signal OS0 to“0” for a predetermined period of time. This causes a latch made of NANDgates 103-0 and 104-0 to yield an output LT0 of “1,” and the outputsignal LD0 is set to “1.” When the input signal BA1 becomes “1,” asignal OS1 is set to “0” for a predetermined period of time. This causesa latch made of NAND gates 103-1 and 104-1 to provide an output LT1 of“1,” and the output LT0 is set to “0.” The output signal LD1 becomes“1,” and the output signal LD0 reaches “0” after a predetermined delaycaused by a delay circuit 105-0. When the input signal BA2 reaches “1,”the output signal LD2 is likewise set to “1” and the output signal LD1becomes “0” after a predetermined delay. When the input signal BA3reaches “1,” the output signal LD3 is likewise set to “1” and the outputsignal LD2 becomes “0” after a, predetermined delay. Finally at time t4when the input signals BA0 through BA3 all become “0,” an output 1 DB ofan OR gate 106 is set to “0.” At this point, a signal LT3 set so far to“1” reaches “0.” The output signal LD3 becomes “0” after a predetermineddelay.

As described, any one of the voltage limiter circuit VDL0 through VDL3is turned on when an active command for activating the nearby memorybank is input, and is turned off when an active command for activatingany other memory bank is input. That is, each voltage limiter circuitdrives the power supply line. The driving force (driving ability) foreach voltage limiter circuit to drive the power supply line may bevariably controlled while the corresponding memory bank is active.Immediately after a bank active command is input, a large current flowsbecause of a sense amplifier operation that is triggered. The currentflowing in that period is supplied from the voltage limiter circuit nearthe bank in question. Because only a current large enough to sustain thevoltage level flows following the sense amplifier activation, either acircuit far away from the bank or the standby voltage limiter circuitVDLS can supply a sufficient current. That is because the outputs of thevoltage limiter circuits are interconnected. Under such a controlscheme, even with a plurality of memory banks activated, only onevoltage limiter circuit besides the VDLS is active most of the time(although two or more voltage limiter circuits may be turned ontemporarily). Naturally, only one voltage limiter circuit is active inaddition to the VDLS in active standby mode. Compared with theconventional setup where all voltage limiter circuits are activatedduring operation, the inventive scheme significantly reduces powerdissipation in the active standby state. Since many synchronous DRAMs inoperation have their memory banks left active to take advantage of thehigh-speed data transfer feature, the drop in active standby currentcontributes appreciably to a reduction in the power dissipation of thesystem as a whole.

Each voltage limiter circuit is turned off for a predetermined period oftime after the input of a bank active command for activating anotherbank for the following reason: a large current flows immediately after abank active command is input. The time during which the large currentflows is from 10 to 30 ns depending on the process technique and designconcept in use. Meanwhile, the intervals at which bank active commandsare continuously input are stipulated for all synchronous DRAMs; and theinternals are usually made of two clock cycles. Illustratively, if theclock frequency is 100 MHz, the intervals are 20 ns long. This meansthat if an active command for the bank 1 is input two clock cycles afterthe input of an active command for the bank 0, there is a possibilitythat a large current is still flowing through the bank 0. Immediatelyturning off the voltage limiter circuit VDL0 at that point will cause alarge current to be supplied from the circuit VDL1 far away from thebank 0. In that case, a voltage drop due to line resistance can triggera malfunction or an operation delay, which can lead to negativeconsequences.

FIG. 7 is another operation waveform chart applicable to the controlcircuit of FIG. 5 in a refresh mode (auto-refresh represented by astandardized SDRAM command REF). Inputting an auto-refresh commandactivates all banks simultaneously. In each bank, a word line indicatedby a refresh counter is activated so as to refresh one-word memorycells. At time t5 when a refresh command (REF) is input, all bank activesignals BA0 through BA3 and the refresh signal RF are set to “1.” Thissets the latch outputs LT0 through LT3 as well as the output signals LD0through LD3 to “1.” When the refresh operation terminates in the chip(at this time no command input is required), the signals RF and BA0through BA3 are set to “0”; the output IDB of the OR gate 106 becomes“0,” the latch outputs LT0 through LT3 reach “0,” and the output signalsLD0 through LD3 are set to “0” after a predetermined delay. That is, thevoltage limiter circuits VDL0 through VDL3 are turned on at the sametime in the refresh mode. The simultaneous circuit activation isintended to supply a current four times as large as that in normaloperation because all banks operate simultaneously in the refresh mode.The standby voltage limiter circuit VDLS is continuously in operation.

The control circuit CTL1 will now be described with reference to thecircuit diagram of FIG. 8 and the operation waveform charts of FIGS. 9through 12. The input signal CKE is a clock enable signal that issupplied externally. PDMB denotes a signal that is set to “0” inpower-down mode or self-refresh mode (to be described later) and is setto “1” otherwise. RF represents a refresh signal that is set to “1” whenthe memory is being refreshed. BA indicates a signal that is set to “1”when a bank active command is input. RD and WR stand for signals thatare set to “1” in a read mode and in a write mode respectively. Theoutput signal LC4 is used to turn on and off the VCL limiter circuitVCL4 as described above (set to “1” for circuit activation and to “0”for deactivation). The output signal LC5 is a signal that turns on andoff the VCL limiter circuits VCL5 and VCL6 as mentioned above (set to“1” for circuit activation and to “0” for deactivation).

FIG. 9 is an operation waveform chart applicable to the control circuitin FIG. 8 when the bank 0 is activated and data are read therefrom. Attime t6 when an active command (ACTV0) for the bank 0 is input, thesignal BA is set to “1” while the command is being input, and an outputDLY1 of the delay circuit 106 reaches “1” for a predetermined period oftime. At time t7 when a read command (READ0) for the bank 0 is input,the signal RD is set to. “1” because a read mode is selected, and anoutput DLY2 of the delay circuit 108 becomes “1.” At time t8 when aprecharge command (PRE0) for the bank 0 is input, the signal RD becomes“0” and the output DLY2 reaches “0” with a slight delay. Because theoutput signal LC5 is obtained by OR'ing DLY1 and DLY2, the signal LC5 isset to “1” immediately after the input of a bank active command as wellas in read mode. The output signal LC4 is always “1” since the clockenable signal CKE is “1.”

As described, the voltage limiter circuits VCL4 through VCL6 and VCLSare all turned on immediately after the input of a bank active commandas well as in a read mode. Otherwise the circuits VCL4 and VCLS aloneare turned on and the circuits VCL5 and VCL6 are turned off. Immediatelyafter the input of the bank active command, a large current flowsbecause the address buffer and row decoders are activated. A largecurrent also flows in a read mode in which the column decoders, mainamplifier and output buffer are activated. In these periods, all voltagelimiter circuits are turned on to supply the large current. In otherperiods, only a small current flows, so that part of the circuits aredeactivated. Specifically, the period in which the circuits VCL5 andVCL6 operate should be made as long as or slightly longer than (to allowfor some margins for fluctuations) the period in which the row decodersand column decoders operate in connection with the activation of a bankand the reading of data therefrom. Under such a control scheme, only onevoltage limiter circuit besides the VCLS is active in an active standbymode (i.e., from bank activation until the input of a read command).Thus compared with the conventional setup where all voltage limitercircuits are turned on in operation, the inventive scheme significantlyreduces power dissipation in an active standby mode.

Whereas the description above has centered on read operations, thedescription also applies to write operations taking place in the samemanner except that the signal WR instead of the signal RD is set to “1.”In write operations, the active standby current may be reduced in thelike manner.

FIG. 10 is another operation waveform chart applicable to the controlcircuit of FIG. 8 in the refresh mode (auto-refresh). As in the case ofFIG. 7, when a refresh command (REF) is input at time t5, the refreshsignal RF is set to “1” as long as the refresh operation is beingperformed. The output signal LC5 is set to “1” for a predeterminedperiod of time by a one-shot pulse generating circuit 114 and a delaycircuit 116, and is thereafter brought to “0.” Since the clock enablesignal CKE is “1,” the output signal LC4 is always “1.” Thus immediatelyafter the start of a refresh operation, all voltage limiter circuitsVCL4 through VCL6 and VCLS are turned on. Later, the circuits VCL5 andVCL6 are turned off. Immediately after the refresh operation hasstarted, a large current flows because of the operation of the rowdecoders and other elements. During that period, all voltage limitercircuits are activated to supply the large current. Thereafter, withonly a small current flowing, part of the circuits are deactivated tominimize power dissipation.

FIG. 11 is another operation waveform chart applicable to the controlcircuit of FIG. 8 in a power-down mode. Power-down mode is a low powerdissipation mode corresponding to a normal standby state of the DRAMs(in which all data lines are precharged to VDL/2). DRAMs have astandardized method for designating the start and end of power-down modeby use of the clock enable signal CKE, among others. This embodimentalso employs the standardized method. At time t9 when a power-down modestart command (PDM Entry) is input, the signal PDMB reaches “0,” whichsets the output signal LC4 to “0.” The signal CKE is set to “1” beforetime tl0 when a power-down mode end command (PDM Exit) is to be input.Thus at time tl0, the output signal LC4 is set to “1.” Because thesignals LC4 and LC5 are set to “0” in a power-down mode, the voltagelimiter circuits VCL4 through VCL6 are all turned off and only thestandby voltage limiter circuit VCLS is turned on. It follows that powerdissipation is minimized while a power-down mode is in effect.

The fall of the signal LC4 is determined by the signal PDMB while itsrise is decided by the clock enable signal CKE for the followingreasons: the specifications of synchronous DRAMs stipulate that apower-down mode be terminated by setting the clock enable signal CKE to“1” at a leading edge of the clock signal CLK. The signal CKE must beset to “1,” earlier than the rise of the clock signal'CLK by a set-uptime (usually 2 to 3 ns). This means that the signal LC4 is raisedearlier if the end of the power-down mode is judged based on the signalCKE than if the end of the mode is determined as per the signal PDMB.Since there is a possibility that a bank active command or a likecommand may be input during the clock cycle immediately after thepower-down mode end command, the voltage limiter circuit VCL4 shouldpreferably be turned on earlier than usual in preparation for thecommand input.

FIG. 12 is another operation waveform chart applicable to the controlcircuit of FIG. 8 in a self-refresh mode. When a self-refresh command isinput, the SDRAM is refreshed periodically according to an internaltimer until a self-refresh end command is entered. At time t11 when aself-refresh start command (SELF Entry) is input, the signal PDMB is setto “0” bringing the output signal LC4 to “0.” When the timer in the chipstarts a refreshing operation at time t12, the signal RF is set to “1.”The output signal LC4 is at “1” while the signal RF is “1.” The outputsignal LC5 remains at “1” for a predetermined Period of time followingarise of the signal RF. The signal CKE is at “1” before time t13 when aself-refresh mode end command (SELF Exit) is to be input. When time t13is reached, the signal LC4 is set to “1.” Thus in a self-refresh mode,the voltage limiter circuits VCL4 through VCL6 are turned on only whenthe refresh operation is being actually performed and are turned offotherwise. Refresh operations are carried out usually at intervals oftens to hundreds of microseconds.

A single refresh operation is completed in tens of ns, which means thatthe duration of the refresh operations accounts for 0.1% or less of alloperation times. Inmost of the remaining operation times, the voltagelimiter circuits VCL4 through VCL6 are all turned off and only thestandby voltage limiter circuit VCLS is turned on. As a result, thepower dissipation in the self-refresh mode is quite limited.

FIG. 13 is a circuit diagram of a typical VCL voltage limiter circuit.The circuit comprises a differential amplifier DA and an output stageFS. The differential amplifier DA is activated when an active commandLCi is “1” (High level). When active, the amplifier DA compares avoltage VFB with the reference voltage VRC, the voltage VFB beingobtained by dividing the output voltage VCL by two P-channel MOStransistors M19 and M20. An output PG of the differential amplifier DAis input to the gate of a P-channel MOS transistor M16 in the outputstage. A drop in the VCL potential leads to a decline in the PGpotential, turning on the MOS transistor M16 to charge the VCL from theVDD. An excess rise in the VCL potential causes a leakage circuit madeup of N-channel MOS transistors M17 and M18 to discharge the VCL. Whenthe active signal LCi is at “0” (Low level), an N-channel MOS transistorM1 is turned off; P-channel MOS transistors M8, M9 and M15 are turnedon; and an N-channel MOS transistor M14 is turned on. This in turnactivates P-channel MOS transistors M4 through M7 and deactivatesN-channel MOS transistors M12 and M13, thereby cutting off all currentsflowing through the differential amplifier DA. In addition, the MOStransistors M16 and M18 in the output stage are turned off. As a result,the power dissipation consisting only of currents flowing through thevoltage dividers M19 and M20 is at a level significantly lower than thatin the operating state. The transistors M19 and M20 are continuouslysupplied with currents to stabilize the level of the voltage VFB inpreparation for the next activation of the differential amplifier DA.

FIG. 14 is a circuit diagram of a typical VDL voltage limiter circuit.The difference between the circuit of FIG. 14 and that of FIG. 13 isthat the setup of FIG. 14 has a reinforced capability of discharging atthe time of an excess rise in the output voltage VDL. In FIG. 14, adifferential amplifier DA has two outputs PG and NG. The output PG isinput to the P-channel MOS transistor M16 as in the circuit of FIG. 13,while the output NG is input to the gate of an N-channel MOS transistorM25. An excess rise in the VDL potential leads to an increase in the NGpotential, which causes the transistor M25 to discharge the VDL. Thearrangement above is incorporated to deal with the situation where theoverdrive technique (see FIG. 3) is utilized, i.e., where the VDLpotential can be raised too high. Meanwhile, although this embodimenthas been described with four banks, the number of banks may be changedas desired as long as the number is an integer. Preferably, the numberof banks should be 2 to the n-th power, “n” being an integer (e.g., 2,4, 8, 16, 32, etc.). Whereas other embodiments to be described belowwill also have four banks, that number of banks is used only forpurposes of illustration and is not limitative of the invention.

The control scheme described above is made possible when the number ofvoltage limiter circuits (excluding standby voltage limiter circuits)generating the first internal voltage VDL is made equal to the number ofbanks configured (four with this embodiment). On the other hand,although this embodiment has three voltage limiter circuits (excludingstandby voltage limier circuits) generating the second internal voltageVCL, the circuit count is optional as long as there are at least twocircuits. That is, one voltage limiter circuit is controlled by thecontrol signal LC4 and the other circuit by the signal LC5. As standbyvoltage limiter circuits, one or more circuits are needed for thevoltage VDL and one or more for the voltage VCL. Preferably, thereshould be only one standby voltage limiter circuit for each of thevoltages VDL and VCL so as to minimize power dissipation in the standbymode.

Second Embodiment

The second embodiment of this invention will now be described. Thelayout of circuits in the second embodiment is the same as that in FIG.1 and will not be described here. FIG. 15 depicts connective relationsbetween the circuits involved. What makes the setup of FIG. 15 differentfrom that of FIG. 4 is that the voltage limiter circuits VDL0 and VDL1are controlled collectively by the active signal LD0 and the circuitsVDL2 and VDL3 by the active signal LD2.

FIG. 16 is a circuit diagram of the voltage limiter control circuitCTL0, and FIG. 17 is an operation waveform chart applicable to thecontrol circuit in FIG. 16. As in the waveform chart of FIG. 6, thewaveforms in FIG. 17 are in effect when active commands to active thebanks 0, 1, 2 and 3 are input at times t0, t1, t2 and t3 respectively,with an all-bank precharge command being entered at time t4. When theactive signal BA0 is first set to “1,” the signal OS0 becomes “0” for apredetermined period of time as with the case in FIG. 6. An output LT0of a latch made of NAND gates 103-0 and 104-0 then becomes “1” and theoutput signal LD0 is set to “1.” When the active signal BA1 is set to“1,” the signal OS1 becomes “0” for a predetermined period of time. Atthis point, the latch outputs LT0 and LT2 remain unchanged. When theactive signal BA2 is set to “1,” the signal OS2 becomes “0” for apredetermined period of time. As a result, the output LT2 of a latchmade of NAND gates 103-2 and 104-2 is set to “1”, and the latch outputLT0 is set to “0.” The output signal LD2 reaches “1,” and the signal LD0becomes “0” following a predetermined delay caused by the delay circuit105-0. When the active signal BA3 is set to “1,” the signal OS3 becomes“0” for a predetermined period of time but the latch outputs LT0 and LT2remain unchanged. At time t4 when all active signals BA0 through BA3 areset to “0,” the output IDB of the OR gate 106 reaches “0.” This sets to“0” the signal LT2 which has been at “1” so far. The output signal LD2reaches “0” after a predetermined delay.

The voltage limiter circuits VDL0 and VDL1 may be grouped into a singlecircuit because they are always turned on and off together. The sameapplies to the circuits VDL2 and VDT3.

In the second embodiment, two voltage limiter circuits are controlledcollectively by a single active signal. For that reason, two voltagelimiter circuits besides the VDLS are turned on most of the time.Although its ability to save power is not as good as that of the firstembodiment, the second embodiment offers the benefit of a simplifiedcontrol circuit structure and used a smaller number of active signals.This feature is particularly advantageous where the number of banks isconsiderably large. For example, a memory with 0.16 banks would need 16active signals if each bank had one voltage limiter circuit controlledindividually. Instead, the second embodiment has the 16 banks groupedinto four circuits that are controlled by only four active signals.

The control circuit CTL1 is the same in structure as that of the firstembodiment (FIG. 8). Thus no further description will be made of thatcircuit.

With the second embodiment, the number of VDL voltage limiter circuits(excluding standby voltage limiter circuits) is made equal to the numberof banks (i.e., four). Since the circuits VDL0 and VDL1 are controlledby the same control signal LD0 and are turned on and off togetherthereby, the two circuits may be grouped into a single circuit. The sameapplies to the circuits VDL2 and VDL3. If VDL voltage limiter circuitscontrolled by the same control signal are grouped into a single circuit,the number of the voltage limiter circuits in use becomes a measure ofthe number of the banks configured.

Third Embodiment

FIG. 18 is a schematic view of a synchronous DRAM practiced as the thirdembodiment of this invention. The difference between the circuit of FIG.18 and that of FIG. 1 is that the setup of FIG. 18 comprises anadditional VDL voltage limiter circuit VDL4. The circuit VDL4 iscontrolled by an active signal LC4 as shown in FIG. 19.

FIG. 20 is a circuit diagram of a voltage limiter control circuit CTL0in the SDRAM of FIG. 18, and FIG. 21 is an operation waveform chartapplicable to the control circuit in FIG. 20. As in the cases of FIGS. 6and 17, active commands to activate the banks 0, 1, 2 and 3 are input attimes t0, t1, t2 and t3 respectively, with an all-bank precharge commandentered at time t4. When the active signal BA0 is first set to “1,” anoutput OS10 of a one-shot pulse generating circuit 108-0 becomes “1” fora predetermined period of time. This causes the output signal LD0 toreach “1” and then to return to “0” with a predetermined delay. When theactive signals BA1, BA2 and BA3 are successively set to “1,” the outputsignals LD1, LD2 and LD3 are brought to “1” respectively for apredetermined period of time each. The signal LC4 is always at “1.”

As a result, each of the voltage limiter circuits VDL0 through VDL3 isturned on only immediately after the active command corresponding to thememory bank nearby is input. The voltage limiter circuit VDL4 iscontinuously active. Immediately after a bank active command is entered,a large current flows because of the sense amplifier operation beinginitiated. The current flowing during that period is derived primarilyfrom the voltage limiter circuit near the bank in question. After thesense amplifier operation, only a small current flows which can besupplied adequately from the voltage limiter circuit VDL4 or VDLS. Undersuch a control scheme, even if a plurality of memory banks are turned on(although two or more voltage limiter circuits are together activatedtemporarily), the only voltage limiter circuits being active most of thetime are VDLS and VDL4. Needless to say, only the voltage limitercircuits VDLS and VDL4 are turned on in the active standby mode.Compared with conventional setups where all voltage limiter circuits areactivated during operation, the power dissipation in the active standbymode is thus reduced considerably.

As is clear from the above description, the power supplying capabilityof the voltage limiter circuit VDL4 is allowed to be smaller than thatof the circuit VDL0, VDL1, VDL2 or VDL3 but should preferably be largerthan that of the circuit VDLS. That is because a current for invertingthe potential of a bit line pair flows in the write mode, the currentbeing supplied from the circuit VDL4 (with the circuits VDL0 throughVDL3 being turned off). Controlled by the active signal LC4, the circuitVDL4 is turned off in power-down mode or in self-refresh mode (see FIGS.11 and 12).

A major advantage of the third embodiment is thus a simplified structureof the control circuit CTL0. The structural simplicity becomes apparentwhen FIGS. 20, 5 and 16 are compared with each other. The controlcircuit CTL1 is the same in structure as that of the first embodiment(FIG. 8). Thus no further description will be made of that circuit. Withthe third embodiment, whenever the number of banks in use is changed,the minimum number of VDL voltage limiter circuits configured (excludingstandby voltage limiter circuits) need only be made equal to the numberof the banks being used plus 1. That is, there should be one circuitcorresponding to each bank and at least one circuit controlled by thesignal LC4.

Fourth Embodiment

FIG. 22 is a schematic view of a synchronous DRAM practiced as thefourth embodiment of this invention. The fourth embodiment ischaracterized in that the control scheme for VDL voltage limitercircuits (typical of the third embodiment) is applied to VCL voltagelimiter circuits. Each of VCL voltage limiter circuits VCL0 through VCL3in the fourth embodiment is located near a corresponding bank. In themiddle of the configured banks are a voltage limiter circuit VCL4 and astandby voltage limiter circuit VCLS. FIG. 23 shows connecting relationsbetween the component circuits of the fourth embodiment. As in the caseof the third embodiment (FIG. 19), VDL voltage limiter circuits VDL0through VDL4 are controlled by active signals LD0, LD1, LD2, LD3 and LC4respectively. The VCL voltage limiter circuits VCL0 through VCL4 arecontrolled by active signals LC0, LC1, LC2, LC3 and LC4 respectively.

The control circuit CTL0 is the same in structure as that of the thirdembodiment (FIG. 20). Thus no further description will be made of thatcircuit. The control circuit CTL1 is described below with reference tothe circuit diagram of FIG. 24 and the operation waveform chart of FIG.25. As in the case of FIG. 9, the setup of FIG. 24 has a bank 0activated and has data read therefrom. An active command to activate thebank 0 is input at time t6, which sets a signal BA0 to “1.” This causesan output OS20 of a one-shot pulse generating circuit 120-0 to become“1” for a predetermined period of time. The output OS20 being “1” causesthe output signal LC0 to reach “1” and then to return to “0” with apredetermined delay caused by a delay circuit 122-0. At time t7 when aread command to read data from the bank 0 is input, the read mode isselected in which the signal RD is set to “1” and all output signals LC0through LC3 are also set to “1.” At time t8 when a precharge command(PRE 0) to precharge the bank 0 is input, the signal RD is brought to“0” and, a ‘little later, the signals LC0 through LC3 are also broughtto “0.” The output signal LC4 is always at “1” because the clock enablesignal CKE is set to “1.”

As a consequence, the voltage limiter circuits VCL0, VCL4 and VCLS areturned on immediately after the input of a bank active command. In theread mode, the circuits VCL0 through VCL4 and VCLS are all turned on;otherwise the circuits VCL4 and VCLS alone are activated. Because alarge current flows immediately after the input of the bank activecommand or when a read mode is in effect, the number of activatedvoltage limiter circuits is increased during such periods in order toprovide a large current. At other times, the current flow is small, sothat the number of active voltage limiter circuits may be reducedaccordingly. Under such a control scheme, only the voltage limitercircuits VCL4 and VCLS are turned on in the active standby mode (frombank activation until the issue of a read command). Compared withconventional setups where all voltage limiter circuits are activatedduring operation, the power dissipation in the active standby mode isthus reduced considerably.

As is clear from the foregoing explanation, the power supplyingcapability of the voltage limiter circuit VCL4 is allowed to be smallerthan that of the circuit VCL0, VCL1, VCL2 or VCL3 but should preferablybe larger than that of the circuit VCLS. That is because a small portionof the circuits (e.g., input buffers for clock signal CLK) are operatingeven in the active standby mode. Controlled by the active signal LC4,the circuit VCL4 is turned off in a power-down mode or in a self-refreshmode (see FIGS. 11 and 12).

Whereas the description above has focused on read operations, thedescription also applies to write operations taking place in the samemanner except that the signal WR instead of the signal RD is set to “1.”In write operations, ’the active standby current may be reduced in alike manner.

With the fourth embodiment, the minimum number of VDL or VCL voltagelimiter circuits configured (excluding standby voltage limiter circuits)need only be made equal to the number of the banks being used plus 1.That is, there should be one circuit corresponding to each bank and atleast one circuit controlled by the signal LC4.

Fifth Embodiment

FIG. 26 is a schematic view of a synchronous DRAM practiced as the fifthembodiment of this invention. The difference between the circuit of FIG.26 and that of FIG. 1 is that the setup of FIG. 26 has an internalsupply voltage VII generated for dedicated use by internal buffers andthat an internal supply voltage VDL for memory arrays is equal in levelto an internal supply voltage VCL for peripheral circuits.Illustratively, the voltages are at the following levels: VDD=3.3 V,VCL=VDL=1.8 V and VII=2.5 V. The input buffers need to have a stablesupply voltage because they are required to meet the specifications ofthe high level VIH and low level VIL regarding input signals. For thatreason, a supply voltage dedicated to the input buffers is generated ina stable manner by voltage limiter circuits VIIO through VII2 and VIIS.Since the input buffers are included in the peripheral circuits CKT asmentioned earlier, the VII line passes through the circuits CKT.

FIG. 27 shows connecting relations between the component circuits of thefifth embodiment. Because the circuit constitution of the fifthembodiment is roughly the same as that of the first embodiment (in FIG.4), the only differences therebetween are described below. The fifthembodiment has the voltage limiter circuits VII0 through VII2 and VIISadded in order to generate the internal supply voltage VII. Thesecircuits generate the voltage VII by referring to a reference voltageVRI generated by a reference voltage generating circuit RVG (not shownin FIG. 26). The circuit VIIS is a circuit which is continuously activeand which provides a small current supplying capability while consuminga limited amount of power. The circuits VIIO through VII2 provide alarge current supplying capability while dissipating a relatively largeamount of power, and are turned on and off by an active signal LC4. Assuch, the circuits VII0 through VII2 are always active except in apower-down mode or in a self-refresh mode.

Because the level of the internal supply voltage VDL is equal to that ofthe internal supply voltage VCL, the VDL and VCL voltage limitercircuits make use of a common reference voltage VRC. There is no standbyVDL voltage limiter circuit VDLS. Instead, the VDL and VCL voltagelimiter circuits are connected by means of a suitable resistor (MOStransistor MC in the case of FIG. 27); Where all banks are inactive, thevoltage limiter circuits VDLO through VDL3 are turned off. At thatpoint, the VDL level is retained by the VCL voltage limiter circuitsthrough the transistor MC. In this manner, where the VDL level is equalto the VCL level, the circuit VDLS may be omitted. The absence of thecircuit contributes to a corresponding reduction in power dissipation ina power-down mode or in a self-refresh mode.

With the fifth embodiment, the number of VDL voltage limiter circuitsand that of VCL voltage limiter circuits (excluding standby voltagelimiter circuits) may be determined in the same manner as with the firstembodiment. Although the fifth embodiment incorporates three VII voltagelimiter circuits (excluding standby voltage limiter circuits), this isnot limitative of the invention. There need only be a minimum of one VIIvoltage limiter circuit. As described, the fifth embodiment ischaracterized by the absence of a standby VDL voltage limiter circuit.

Although the invention has been shown as being applied to synchronousDRAMs, this is not limitative of the invention. The invention may alsobe applied to semiconductor memories such as DDR (double data rate)SDRAMs, Synclink-DRAMs and Rambus-DRAMs which have a plurality of bankseach and whose operations are designated by commands regardless ofspecific command formats. It is also possible to apply the invention toa single-chip semiconductor device comprising a similar DRAM, logiccircuits, a CPU and other elements in a mixed fashion. The invention isparticularly advantageous when applied to a semiconductor device with amemory having data read into its sense amplifiers and latched therein,as in the case of the active standby mode for SDRAMs.

Although the description above has largely focused on memory banks ofthe synchronous SDRAM, this is not limitative of the invention. Theinvention may also be applied to memory arrays (memory blocks) of DRAMs,SRAMs and other memories, as well as to logical function-equippedmemories including DRAMs, SRAMs and other memories.

The voltage limiter circuits discussed above may be replaced by variousvoltage generating circuits for generating a boosted voltage (Vpp), asubstrate voltage (Vbb) and a plate voltage (Vp1). The power dissipationof these voltage generating circuits is reduced when the operatingstatus of the voltage generating circuits (including a boosted voltagegenerator, a substrate voltage generator, a plate voltage generator) iscontrolled suitably corresponding to the operating state of the memoryarrays (memory blocks, banks, etc.) of memories such as SDRAMs, DRAMsand SRAMs.

Thus, as described, on-chip voltage limiter circuits in the activatestandby mode may have their power dissipation reduced according to theinvention.

While preferred embodiments of the invention have been described usingspecific terms, such description is for illustrative purposes only, andit is to be understood that changes and variations may be made withoutdeparting from the spirit or scope of the claims that follow.

1. A semiconductor device comprising: first and second memory banksincluding a plurality of sense amplifiers and adapted to be activated byfirst and second commands, respectively; a power supply line adapted tosupply a predetermined voltage to the plurality of sense amplifiers ofsaid first and second memory banks and arranged in a mesh manner abovethe first and second memory banks; and first and second power supplycircuits having respective output nodes connected to said power supplyline, said first and said second power supply circuits being adapted togenerate said predetermined voltage; wherein said first power supplycircuit are adapted to start generating said predetermined voltage inresponse to said first command; and wherein, when said second command isinputted while said first memory bank is active, said first power supplycircuit is adapted to stop generating said predetermined voltage inresponse to said second command and said second power supply circuit isadapted to start generating said predetermined voltage in response tosaid second command.
 2. The semiconductor device according to claim 1,wherein, when said second command is inputted while said first memorybank is active, said first power supply circuit is adapted to stopgenerating said predetermined voltage upon elapse of a predeterminedperiod of time after said second power supply circuit has startedgenerating said predetermined voltage.
 3. The semiconductor deviceaccording to claim 1, wherein, when a third command which designates todeactivate said first and said second memory banks is inputtedsimultaneously, one of said first and said second power supply circuitscurrently generating said predetermined voltage is adapted to stopgenerating said predetermined voltage in response to said third command.4. The semiconductor device according to claim 1, wherein, when a fourthcommand which designates to refresh a plurality of memory cells includedin said first and said second memory banks is inputted, said first andsaid second power supply circuits are adapted to start generating afirst voltage in response to said fourth command, and wherein said firstand said second power supply circuits are adapted to stop generatingsaid first voltage upon elapse of a predetermined period of time.